Utility companies conventionally bury their utility lines beneath the surface of the earth in order to provide additional protection for the lines and to prevent the lines from becoming an undesirable addition to the landscape. This is true both for urban utility lines, such as water, sewer, gas, and electrical, and for cross country lines for transporting oil, chemicals, natural gas, and electricity, as well as other products. Even though the lines are more protected beneath the surface of the ground, they are still subject to being severed either by natural causes or by careless excavation. Further, buried utility lines must be located and exposed occasionally for inspection and repair.
It has become customary to lay a wire or tape above a utility line to facilitate location of the line and to give an early warning that the line is near. If the wire or tape is electrically conducting, a metal detector may be utilized to indicate the general vicinity of the line, so that excavation can proceed with the necessary caution to avoid accidental damage or severance of the line. U.S. Pat. No. 3,282,057, of A. W. Prosser entitled UNDERGROUND LINE AND METHOD OF INSTALLING SAME describes the installation and use of such tape.
The warning tape presently used in the utility industry in association with buried lines is of one of two types. The tape is either a narrow thin plastic strip with a sign or legend printed on at least one surface of the tape in a repetitive pattern, or it is a narrow, thin assembly of three layers with a sign on at least one of the sides of the assembly, such as shown in U.S. Pat. No. 4,623,282 of Gordon H. Allen. In the latter construction, the central layer is an electrically conducting foil which is secured by adhesive, heat or coating to the two outer layers which are thin plastic sheets.
Since buried lines are generally a distance of the order of feet below the surface of the earth, it is conventional practice to use earth excavating equipment to lay such lines or to expose such lines for inspection or repair. When excavating equipment approaches a buried line which is protected by a warning tape, either intentionally or accidentally, it is the intention of the line constructor that the excavator encounter the warning tape before he encounters the line, and accordingly, a legend is printed on the warning tape to advise the excavator of the precise risk undertaken by continued excavation. The tape thus carries a sign which may even be a legal notice to the excavator of liability for damage to the adjacent utility line.
The sign on the tape is of little value if it cannot be read. Unfortunately, many tapes distort on excavation or assume a contour effectively masking the information of the sign on the tape. Warning tapes are almost universally in the form of thin plastic strips. Such strips tend to roll or twist into a rope configuration if placed under tension, and in this configuration the sign is partly obscured and illegible. Further, excavation is generally performed with a bucket type digger such as a trench digger, drag line or Bobcat. The inventor has found that a bucket type digger affects the legibility of the sign on the tape differently depending upon the direction of approach to the buried marking tape. If the digger approaches the tape perpendicularly from the side, the walls of the trench dug by the digger are not appreciably distorted and the tape is held sufficiently securely by the earth in these walls to cause the bucket of the digger to shear the warning tape, thus leaving a short section of warning tape in the bucket or in the earth discharged from the bucket. Sections of warning tape sheared in this manner may be hard to locate for the operator, but they have not been subject to tension along the axis of the tape and will not be unduly distorted but tend to be legible.
If the bucket type digger approaches the tape along a path on or near the axis of the tape in a parallel direction to the installation, the earth surrounding the tape becomes loosened by the digger and the digger tends to grab the tape and place a section of the tape under axial tension, often with disastrous results as far as the preservation of legibility of the sign on the tape is concerned. The axial tension tends to stretch the tape, distorting the sign, and in the end tearing the tape erratically.
It is not feasible to bury a tape which is sufficiently strong to resist the deleterious affects of the digging operation described above. If the bucket approaches the tape normal to its elongation axis, the sharp walls of the bucket working against the surfaces of the trench being dug by the digger cut the tape, and it would require a very thick tape, if made of plastic, or a very strong tape to resist the cutting action of the bucket. Further, the tape tends to be legible when cut from this direction. To provide a tape which will not stretch or tear when the digger excavates generally parallel to the longitudinal axis of the tape increases the cost of the tape unjustifiably.